Lithium batteries serve as popular and potential power sources for portable electronic devices, Hybrid Electric Vehicle (HEV) and Plugged in Hybrid Electric Vehicles (PHEVs). Among the promising cathode candidates that are widely used in rechargeable lithium batteries, mixed metal oxide cathodes assume greater importance due to advantages such as high voltage, high capacity and structural stability. In this regard, candidates viz., LiNi0.5Mn0.5O2 (552), LiNi0.33Mn0.33Co0.33O2 (333) and LiNi0.4Mn0.4Co0.2O2 (442) are considered as potential cathode materials, wherein synergistic effect of individual transition metal cations that improves the electrochemical behavior of such solid solution cathodes. Here again, solid solution consisting of cobalt is found to offer better lattice ordering, which is responsible for improved cyclability and extended electrochemical window possibilities. Among the high voltage cathodes, the less studied LiNi0.4Mn0.4Co0.2O2 cathode has been chosen for the present study.
Surface coating has been proven to be effective for improving the capacity retention, rate capability and even thermal stability of cathode materials for lithium-ion batteries. To date, few additives have been reported to provide a protective passivation film for cathodes. Alternatively, surface coating has been reported to extend the performance of cathode materials by either changing the surface chemistry or by protecting the surface by undesirable side reactions. The coating materials investigated to date include carbon, metal oxides, metal carbonates metal aluminates, metal phosphates, metal fluorides, metal oxyfluorides, metal hydroxides and Li2O.2B2O3 glass, as well as cathode materials with lower reactivity towards non-aqueous electrolytes. However, the title compound has been studied only for its surface modification with CNT and no other additives.
Reference may be made C. Ban, Z. li, Z. Wu, M. J. Kirkham, Le Chen, Y. S. Jung, E. A. Payzant, Y. Yan, M. S. Whittingham, and A. C. Dillon, Adv. Energy Mater. 1 (2011) 58-62 wherein LiNi0.4Mn0.4Co0.2O2 (95 wt. %) and Single Wall Carbon Nanotubes (SWCNTs) (5 wt. %) were mixed and suspended in deionized water by using a 1% concentration of sodium dodecyl sulfate as the surfactant. The resulting film was rinsed with deionized water before transferring to the Al foil employed as the current collector. The study demonstrated high-rate capability of layered LiNi0.4Mn0.4Co0.2O2 by fabricating an electrode composed of 5 wt. % SWCNTs and 95 wt. % LiNi0.4Mn0.4Co0.2O2. Raman spectroscopy evidences the existence of very strong interaction between the surface of some of the SWCNTs and the surface of LiNi0.4Mn0.4Co0.2O2. This strong surface connectivity ensures the fast diffusion of ions and electrons during cycling, resulting in a sustainable capacity at high rates for extended cycles. The drawbacks are carbon nanotubes (SWCNTs) could be employed as a flexible net, enabling reversible cycling for high volume expansion materials. However, the same is not suitable for cathode material which does not undergo volume expansion. Similarly, materials that suffer from poor electrical conductivity and surface over-charge/over-discharge causing capacity fade, especially at high rate only would require modification using CNT, which is not the case with the currently chosen 442 cathode. Further, SWCNTs are expensive and is too difficult to coat CNT uniformly on Sub-micron particles of such compounds.
Reference may be made to J. Cho, C.-S. Kim, Sang-Im Yoob, Electrochem. Solid-State Lett. 3 (2000) 362, wherein sol-gel coating of LiCoO2 by SnO2 and subsequent heat-treatment at relatively low temperatures of 400 and 500° C. greatly improve the structural stability, retaining 86 and 84% respectively, of their initial capacities after 47 cycles between 4.4 and 2.75 V at the 0.5 C rate. The drawbacks are the voltage window under investigation is very narrow in range of 4.4 and 2.75 V. Here again, 600° C. that shows uniform Sn distribution throughout the particles exhibits undesirable phase transition during cycling, because solid solution is formed due to the high temperature treatment at 600° C. So it is necessary to optimize the coating temperature, which should normally be slightly lower than the solid solution formation temperature.
Reference may be made to A. M. Kannan, A. Manthiram, Electrochem. Solid-State Lett. 5 (2002) A167-A169, wherein surface modification of LiMn2O4 spinel oxide with LixCoO2, LiNi0.5Co0.5O2, Al2O3, and MgO has been reported using a chemical processing procedure followed by heat-treatment at 300-800° C. The surface/chemically modified samples showed much better capacity retention at both 25 and 60° C. than does the unmodified LiMn2O4 (˜41% fade in 100 cycles at 60° C.). The drawbacks are coating with crystalline cathode material LiNi0.5Co0.5O2 would lead to non uniform coating and partial substitution of one of the metal cations by the added surface modifier. Therefore, it would not serve the purpose of protecting the layer of active material from the undesirable side reactions due to its contact with the electrolyte.
Reference may be made to J. Liu, A. Manthiram, J. Mater. Chem. 20 (2010) 3961, wherein enhanced electrochemical performances of the high capacity layered oxide solid solution Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode is reported. Basically, the surface has been modified with 2 wt. % Al2O3, 2 wt. % RuO2, and 1 wt. % Al2O3+1 wt. % RuO2. The surface-modified samples exhibit much improved electrochemical performances, particularly the 1 wt. % Al2O3+1 wt. % RuO2 coated sample exhibits the highest discharge capacity and rate capability. The drawback is that one of the surface modifiers in the present study i.e, RuO2 is very expensive and the synergistic effect of Al2O3 and RuO2 is only advantageous. Reference may be made to Y. Jung, A. S. Cavanagh, A. C. Dillon, M. D. Groner, S. M. George, S.-H. Lee, J. Electrochem. Soc., 157(2010) A75-A81, wherein Atomic Layer Deposition (ALD) has been used to prepare pinhole-free surface coating layers on LiCoO2 cathode material with precise control on the thinness of the coating layer down to 0.1 nm. Recently, Lee et al also reported that 2-6 atomic layers of Al2O3 coating significantly improved the capacity retention of LiCoO2. However, thicker coating resulted in dramatic interfacial impedance increase and hence deteriorated the electrochemical performance of the LiCoO2. The drawbacks are ALD process (Atomic Layer Deposition) is not cost effective and requires special expertise to have a control over the thickness of coated film.
Reference may be made to A. Manthiram et al. surface and bulk modified high capacity layered oxide cathodes with low irreversible capacity loss, U.S. Pat. No. 7,678,503B2 wherein, the invention provides a method of modifying a layered oxide (1−x)Li[Li1/3Mn2/3]O2-xLi[Mno.5-yNio.5-yCO2y]O2 cathode with metal oxides. Herein, the irreversible capacity loss decreases from 75 to 41 mAh g−1 and the discharge capacity increases from 253 to 285 mAh g−1 after surface modification in the case of Li[Li0.2Mn0.54Ni0.13Co0.13]O2. Similarly, irreversible capacity loss decreases from 63 to 38 mAh g−1 and the discharge capacity increases from 227 to 250 mAh g−1 in the case of Li[Li0.2Mn0.54Ni0.13Co0.13]O2. In contrast, while the irreversible capacity loss decreases significantly from 60 to 30 mAh g−1, the capacity increases only slightly from 249 to 254 mAh g−1 in the case of the cobalt-free Li[Li0.17Mn0.58Ni0.2]O2 cathode. Thus the surface modification offers the advantage of increasing the discharge capacity significantly. All the surface modifications reduce the irreversible capacity loss. The drawback is that the irreversible capacity loss observed even after surface modification was above 30 mAh g−1, which is high.
As it is well known that the performance of a lithium-ion battery has a major dependence on the electrochemical performance of cathode material (cathode-limited battery system), need to improve the performance of battery active cathode materials assumes greater importance. The best and easiest way to improve the performance of a cathode is to modify the surface of the cathode, especially with respect to oxide and phosphate analogs of carefully chosen metals. ABO2 oxides in general have greater tendency to dissolve in the non-aqueous electrolyte medium and suffer from inferior chemical and/or structural stability, cation-mixing originated off-stoichiometry and lack of hexagonal ordering. Poor cycling stability at extended potential windows above 4.5 V is yet another prime issue that needs to be addressed by suitable modification techniques. Method of surface modification, type, nature, amount and ratio of modifier as a function of pristine cathode matrix, temperature, dwell time, mode (in-situ or ex-situ) of addition and the required type of modification (perfect coating/continuous/discreate coating) are certain parameters, which are to be considered in line with the requisites to improve the electrochemical performance of a cathode using surface modification. Above all, the cathode being selected for modification plays a significant role, wherein properties of the cathode material are better judged by the type of synthesis method adopted, type and ratio of precursors chosen, mode of furnace heating, dwell time, pre- and/or post treatment protocols and synthesis methodology based additives deployed to improve the reaction efficiency of the synthesis approach.
The intriguing point is that the LiNi0.4Mn0.4Co0.2O2 (442) cathode has not been studied in detail for the possible extent of improvement in electrochemical properties. Certain modifiers, viz Al2O3, ZnO, MgO, SnO2, TiO2 and ZrO2 have been discussed so far in the literature, that too with similar category ABO2 oxide cathodes with an exception of 442 compound. Therefore, MxOy, wherein M represents any one of the metals such as Cr, Zn, Bi, Al, In, Mg and Zr are introduced for the first time as surface modifiers with LiNi0.4Mn0.4Mn0.4O2 (442) powder. The select oxides impart the desired Hydrofluoric Acid (HF) scavenging effect, suppressed phase transition and combat the blocking of certain active sites that serve as catalytic sites for electrolyte decomposition. Particularly, Al2O3, Bi2O3 and In2O3 additives demonstrate themselves as suitable surface modifiers, with a special reference to 442 cathode.
The HF scavenging effect of Al2O3, Bi2O3, In2O3 or MxOy as per the mechanism given below has been found to be better in a way that the normally observed capacity loss is minimized with the above mentioned coating.Al2O3+6HF→2AlF3+3H2O  (1)Bi2O3+6HF→2BiF3+3H2O  (2)In2O3+6HF→2InF3+3H2O  (3)Cr2O3+6HF→2CrF3+3H2O  (4)ZrO2+4HF→ZrF4+2H2O  (5)ZnO+2HF→ZnF2+H2O  (6)MgO+2HF→MgF2+H2O  (7)Oy+nHF→xMFn/x+yH2O  (8)Further the number of oxide ion vacancies that remains after first charging has been increased in a favorable manner due to which the initial irreversible capacity loss is minimized significantly in the current investigation. In presence of HF scavenger, which is in general a Lewis base, part of HF in the electrolyte will be neutralized and the acidity of the electrolyte will be reduced. In other words, residual HF in the electrolyte will preferentially react with the added HF scavenger and hence delay the corrosion of cathode electrode materials in the acidic electrolyte. Therefore, one can reasonably expect that the addition of HF scavenger would improve the capacity retention of lithium-ion cells for a relatively short time, as widely reported in the literature. Meanwhile, the metal fluorides as byproducts of reactions 1-8 are generally insoluble in the non-aqueous electrolytes and potentially act as another protective layer to suppress the corrosion of cathode materials. In short, the active LiNi0.4Co0.4Mn0.4O2 cathode surface is totally protected from the undesirable side reactions, facilitated mainly by the HF scavenging effect of metal oxide (MxOy) modifier. Similarly, unlike the reported synthesis methods such as solid-state, combustion and sol-gel, Cetyl Trimethyl Ammonium Bromide (CTAB) co-assisted sol-gel method has been explored for the first time to prepare 442 compound. Further, synergistic effect of synthesis method and the role of select modifiers in improving the electrochemical properties of 442 compound in terms of improved specific capacity, high rate capability, high voltage stability, reduced irreversible capacity behavior have been investigated in detail for demonstration and recommendation of the same for application in rechargeable lithium batteries.
Technical problem to be solved by Invention is that the layered LiNi0.4Mn0.4Co0.2O2 (442), belonging to a solid solution series of LiNiyMnyCo1-2O2 has been surface modified by MxOy type of metal oxide modifier to combat the initial irreversible capacity loss issue, qualify the cathode for high voltage and high rate application.